Subsurface oil reservoirs are promising and important sources for the generation and collection of increasing amounts of methane gas as an energy source. In particular, since at best about 40% of oil is recoverable from oil reservoirs, methods for the conversion of the remaining oil in the reservoirs to clean fuels such as methane are of great interest. Microbes are responsible for biodegrading conventional crude oil to intermediates that are converted by other microbes (methanogens) into methane leaving behind heavy oil, which can be then further biodegraded. The processes of heavy oil formation and methane gas generation (methanogenesis) occur over geological time scales of millions of years. Microbial conversion of oil to methane involves a variety of multi-step pathways and a consortium of microorganisms working together in concert. Pathways include fermentation of oil to acetate, CO2 and hydrogen, as well as microbial oxidation of acetate to CO2 and hydrogen and subsequent microbial reduction of CO2 to methane with hydrogen. Other pathways include direct fermentation of acetate to methane. Each of these biochemical transformations are carried out by specific types of microbes and often take place in the face of competing reactions such as conversion of methane and the acetate and hydrogen intermediates to other products such as H2S, H2O, and CO2.
It has been recognized that the naturally slow methanogenic biodegradation process can be accelerated to enhance methane production in the reservoir. For example, as described in WO 2005/115649 entitled “Process for Stimulating Production of Methane from Petroleum in Subterranean Formations”, techniques are described for injecting one or more agents into a reservoir in which methanogenic microbial consortia are present to modify the reservoir environment to promote in situ microbial degradation of petroleum, promote microbial generation of methane, and to demote in situ microbial degradation of methane.
While that reference describes generic procedures for enhanced methane production, there is still a need for improved methods for stimulating methane production to maximize the potential of subsurface oil reservoirs as sources of methane. Adding the proper amounts and types of agents and nutrients to optimally stimulate methane production from the particular petroleum components found in an oil reservoir remains a considerable challenge in view of the variety of microbes and pathways involved, including microbes that may be present in the reservoir but that do not participate or are deleterious to methane production. For example, stimulation and the growth of non-methanogens may out-compete methanogens for common intermediates essential to methane production or drive microbial transformations to products (e.g. CO2, H2S, etc.) other than methane.